Abstract/Summary

1. Hydroacoustics (2002–04) and long-term oxygen data (1969–2004) have been used in conjunction to examine the habitat of Arctic charr in the north and south basins of Windermere, U.K., a temperate lake subjected to cultural eutrophication and subsequent nutrient management.
2. Since 1969 there has been a gradual decline in the oxygen concentration in the bottom waters of both basins of 0.03–0.04 mg L−1year−1, resulting in up to 43% of the volume of the south basin having an oxygen concentration <5 mg L−1 in the early autumn.
3. Hydroacoustic data indicate that most Arctic charr routinely avoid the upper 10 m of the water column irrespective of temperature, with the implication that an observed gradual warming of the lake has not yet directly impacted upon their habitat.
4. In recent years there has been a behavioural response of the Arctic charr population to migrate vertically to avoid oxygen concentrations <2.3–3.1 mg L−1. Further, the depth of the lower bound of the Arctic charr population is shown to be highly correlated with the deep water oxygen concentration throughout the year prior to autumnal overturn.